Data from: Statistical analysis of dental variation in the Oligocene equid Miohippus (Mammalia, Perissodactyla) of Oregon

As many as eight species of the “anchitherine” equid Miohippus have been identified from the John Day Formation of Oregon, but no statistical analysis of variation in these horses has yet been conducted to determine if that level of diversity is warranted. Variation of the anterior-posterior length and transverse width of upper and lower teeth of Turtle Cove Member Miohippus was compared to that of M. equinanus, Mesohippus bairdii, Equus quagga, and Tapirus terrestris using t tests of their coefficients of variation (V). None of the t tests are significant, indicating that the variation seen in Turtle Cove Miohippus is not significantly different from any of the populations of other perissodactyls examined in this study. Data also indicate that Mesohippus is present in the Turtle Cove Member. Additionally, hypostyle condition, used to diagnose all species of Miohippus, was found to be related to stage of wear using an ordered logistic regression. Only two species of equid, one Miohippus and one Mesohippus, in the Turtle Cove Member can be identified, therefore only Miohippus annectens, the genotype and first species described from the region, can be recognized as the sole Miohippus species known from the Turtle Cove assemblage. There are insufficient data to determine which species of Mesohippus is present. The dependence of hypostyle condition on crown height in Miohippus implies that wear stage must also be considered in investigations of dental morphology in the “Anchitheriinae.”

迄今已从美国俄勒冈州约翰·戴组（John Day Formation）中鉴定出多达8种锚马亚科（Anchitheriinae）马科（equid）动物中新马属（Miohippus）物种，但目前尚未开展相关统计分析以验证该类群的形态变异是否支持如此高的物种多样性水平。本研究通过变异系数（coefficient of variation, V）的t检验，对比了海龟湾段（Turtle Cove Member）中新马属上下颌牙齿的前后长度与横向宽度的变异程度，并将其与M. equinanus、贝氏中马（Mesohippus bairdii）、平原斑马（Equus quagga）以及低地貘（Tapirus terrestris）进行了比较。所有t检验结果均未达到显著水平，表明海龟湾段中新马属的形态变异与本研究中涉及的其他奇蹄目（perissodactyls）类群种群均无显著差异。数据同时显示，海龟湾段地层中存在中马属（Mesohippus）化石。此外，本研究通过有序逻辑回归（ordered logistic regression）分析发现，用于诊断所有中新马属物种的舌侧附尖（hypostyle）形态特征，与牙齿磨损阶段存在显著关联。海龟湾段地层中仅可鉴定出2种马科动物，分别为1种中新马属与1种中马属，因此仅可将连接中新马（Miohippus annectens）——该区域首个被描述的模式种——认定为海龟湾化石组合中已知的唯一中新马属物种。目前尚无足够数据可确定该地层中发现的中马属具体物种。中新马属舌侧附尖形态与齿冠高度的相关性表明，在锚马亚科（Anchitheriinae）的牙齿形态学研究中，必须同时将牙齿磨损阶段纳入考量因素。